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SubjectRe: [PATCH v6 10/10] mm: reparent slab memory on cgroup removal
On Tue, Jun 04, 2019 at 07:44:54PM -0700, Roman Gushchin wrote:
> Let's reparent memcg slab memory on memcg offlining. This allows us
> to release the memory cgroup without waiting for the last outstanding
> kernel object (e.g. dentry used by another application).
>
> So instead of reparenting all accounted slab pages, let's do reparent
> a relatively small amount of kmem_caches. Reparenting is performed as
> a part of the deactivation process.
>
> Since the parent cgroup is already charged, everything we need to do
> is to splice the list of kmem_caches to the parent's kmem_caches list,
> swap the memcg pointer and drop the css refcounter for each kmem_cache
> and adjust the parent's css refcounter. Quite simple.
>
> Please, note that kmem_cache->memcg_params.memcg isn't a stable
> pointer anymore. It's safe to read it under rcu_read_lock() or
> with slab_mutex held.
>
> We can race with the slab allocation and deallocation paths. It's not
> a big problem: parent's charge and slab global stats are always
> correct, and we don't care anymore about the child usage and global
> stats. The child cgroup is already offline, so we don't use or show it
> anywhere.
>
> Local slab stats (NR_SLAB_RECLAIMABLE and NR_SLAB_UNRECLAIMABLE)
> aren't used anywhere except count_shadow_nodes(). But even there it
> won't break anything: after reparenting "nodes" will be 0 on child
> level (because we're already reparenting shrinker lists), and on
> parent level page stats always were 0, and this patch won't change
> anything.
>
> Signed-off-by: Roman Gushchin <guro@fb.com>
> ---
> include/linux/slab.h | 4 ++--
> mm/list_lru.c | 8 +++++++-
> mm/memcontrol.c | 14 ++++++++------
> mm/slab.h | 23 +++++++++++++++++------
> mm/slab_common.c | 22 +++++++++++++++++++---
> 5 files changed, 53 insertions(+), 18 deletions(-)
>
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 1b54e5f83342..109cab2ad9b4 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -152,7 +152,7 @@ void kmem_cache_destroy(struct kmem_cache *);
> int kmem_cache_shrink(struct kmem_cache *);
>
> void memcg_create_kmem_cache(struct mem_cgroup *, struct kmem_cache *);
> -void memcg_deactivate_kmem_caches(struct mem_cgroup *);
> +void memcg_deactivate_kmem_caches(struct mem_cgroup *, struct mem_cgroup *);
>
> /*
> * Please use this macro to create slab caches. Simply specify the
> @@ -638,7 +638,7 @@ struct memcg_cache_params {
> bool dying;
> };
> struct {
> - struct mem_cgroup *memcg;
> + struct mem_cgroup __rcu *memcg;
> struct list_head children_node;
> struct list_head kmem_caches_node;
> struct percpu_ref refcnt;
> diff --git a/mm/list_lru.c b/mm/list_lru.c
> index 0f1f6b06b7f3..0b2319897e86 100644
> --- a/mm/list_lru.c
> +++ b/mm/list_lru.c
> @@ -77,11 +77,15 @@ list_lru_from_kmem(struct list_lru_node *nlru, void *ptr,
> if (!nlru->memcg_lrus)
> goto out;
>
> + rcu_read_lock();
> memcg = mem_cgroup_from_kmem(ptr);
> - if (!memcg)
> + if (!memcg) {
> + rcu_read_unlock();
> goto out;
> + }
>
> l = list_lru_from_memcg_idx(nlru, memcg_cache_id(memcg));
> + rcu_read_unlock();
> out:
> if (memcg_ptr)
> *memcg_ptr = memcg;
> @@ -131,12 +135,14 @@ bool list_lru_add(struct list_lru *lru, struct list_head *item)
>
> spin_lock(&nlru->lock);
> if (list_empty(item)) {
> + rcu_read_lock();
> l = list_lru_from_kmem(nlru, item, &memcg);
> list_add_tail(item, &l->list);
> /* Set shrinker bit if the first element was added */
> if (!l->nr_items++)
> memcg_set_shrinker_bit(memcg, nid,
> lru_shrinker_id(lru));
> + rcu_read_unlock();

AFAICS we don't need rcu_read_lock here, because holding nlru->lock
guarantees that the cgroup doesn't get freed. If that's correct,
I think we better remove __rcu mark and use READ_ONCE for accessing
memcg_params.memcg, thus making it the caller's responsibility to
ensure the cgroup lifetime.

> nlru->nr_items++;
> spin_unlock(&nlru->lock);
> return true;
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c097b1fc74ec..0f64a2c06803 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3209,15 +3209,15 @@ static void memcg_offline_kmem(struct mem_cgroup *memcg)
> */
> memcg->kmem_state = KMEM_ALLOCATED;
>
> - memcg_deactivate_kmem_caches(memcg);
> -
> - kmemcg_id = memcg->kmemcg_id;
> - BUG_ON(kmemcg_id < 0);
> -
> parent = parent_mem_cgroup(memcg);
> if (!parent)
> parent = root_mem_cgroup;
>
> + memcg_deactivate_kmem_caches(memcg, parent);
> +
> + kmemcg_id = memcg->kmemcg_id;
> + BUG_ON(kmemcg_id < 0);
> +
> /*
> * Change kmemcg_id of this cgroup and all its descendants to the
> * parent's id, and then move all entries from this cgroup's list_lrus
> @@ -3250,7 +3250,6 @@ static void memcg_free_kmem(struct mem_cgroup *memcg)
> if (memcg->kmem_state == KMEM_ALLOCATED) {
> WARN_ON(!list_empty(&memcg->kmem_caches));
> static_branch_dec(&memcg_kmem_enabled_key);
> - WARN_ON(page_counter_read(&memcg->kmem));
> }
> }
> #else
> @@ -4675,6 +4674,9 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
>
> /* The following stuff does not apply to the root */
> if (!parent) {
> +#ifdef CONFIG_MEMCG_KMEM
> + INIT_LIST_HEAD(&memcg->kmem_caches);
> +#endif
> root_mem_cgroup = memcg;
> return &memcg->css;
> }
> diff --git a/mm/slab.h b/mm/slab.h
> index 7ead47cb9338..34bf92382ecd 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -268,7 +268,7 @@ static inline struct mem_cgroup *memcg_from_slab_page(struct page *page)
>
> s = READ_ONCE(page->slab_cache);
> if (s && !is_root_cache(s))
> - return s->memcg_params.memcg;
> + return rcu_dereference(s->memcg_params.memcg);

I guess it's worth updating the comment with a few words re cgroup
lifetime.

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